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Network Layer
•
Network applications and services on one end device can communicate with applications and services running on another end device. How is this data communicated across the network in an efficient way?
•
The protocols of the OSI model network layer specify addressing and processes that enable transport layer data to be packaged and transported. The network layer encapsulation enables data to be passed to a destination within a network (or on another network) with minimum overhead.
•
This chapter focuses on the role of the network layer. It examines how it divides networks into groups of hosts to manage the flow of data packets within a network. It also covers how communication between networks is facilitated. This communication between networks is called routing.
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The Network Layer
• Addressing end devices
- In the same way that a phone has a unique telephone number, end devices must be configured with a unique IP address for identification on the network. An end device with a configured IP address is referred to as a host.
• Encapsulation - The network layer receives a protocol data unit (PDU) from the
transport layer. In a process called encapsulation, the network layer adds IP header information, such as the IP address of the source (sending) and destination (receiving) hosts. After header information is added to the PDU, the PDU is called a packet.
• Routing - The network layer provides services to direct packets to a destination host on
another network. To travel to other networks, the packet must be processed by a router.
The role of the router is to select paths for and direct packets toward the destination host in a process known as routing. A packet may cross many intermediary devices before reaching the destination host. Each route the packet takes to reach the destination host is called a hop.
• De-encapsulation
- When the packet arrives at the network layer of the destination host, the host checks the IP header of the packet. If the destination IP address within the header matches its own IP address, the IP header is removed from the packet. This process of removing headers from lower layers is known as de-encapsulation. After the packet is de-encapsulated by the network layer, the resulting Layer 4 PDU is passed up to the appropriate service at the transport layer.
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Network Layer Protocol
•
There are several network layer protocols in existence; however, only the following two are commonly implemented
•
Internet Protocol version 4 (IPv4)
•
Internet Protocol version 6 (IPv6)
•
Other legacy network layer protocols that are not widely used include:
•
Novell Internetwork Packet Exchange (IPX)
•
AppleTalk
•
Connectionless Network Service (CLNS/DECNet)
•
Discussion of these legacy protocols will be minimal.
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Characteristics of the IP Protocol
•
IP is the network layer service implemented by the TCP/IP protocol suite.
•
IP was designed as a protocol with low overhead. It provides only the functions that are necessary to deliver a packet from a source to a destination over an interconnected system of networks. The protocol was not designed to track and manage the flow of packets. These functions, if required, are performed by other protocols in other layers.
•
The basic characteristics of IP are:
•
Connectionless - No connection with the destination is established before sending data packets.
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Best Effort (unreliable) - Packet delivery is not guaranteed.
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Media Independent - Operation is independent of the medium carrying the data.
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IP - Connectionless
•
IP is connectionless and, therefore, requires
no initialexchange of control information to establish an end-to-end connection before packets are forwarded. IP also does not require additional fields in the protocol data unit (PDU) header to maintain an established connection. This process greatly reduces the overhead of IP. However, with no pre- established end-to-end connection, senders are unaware whether destination devices are present and functional when sending packets, nor are they aware if the destination receives the packet, or if they are able to access and read the packet.
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IP - Based Effort Delivery
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IP - Media Independent
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Encapsulating IP
•
IP encapsulates, or packages, the transport layer segment by adding an IP
header. This header is used to deliver the packet to the destination host. The IP header remains in place from the time the packet leaves the network layer of the source host until it arrives at the network layer of the destination host.
•
The process of encapsulating data layer by layer enables the services at the different layers to develop and scale without affecting other layers. This means that transport layer segments can be readily packaged by IPv4 or IPv6 or by any new protocol that might be developed in the future.
•
Routers can implement these different network layer protocols to operate
concurrently over a network to and from the same or different hosts. The routing performed by these intermediate device only considers the contents of the
packet header that encapsulates the segment. In all cases, the data portion of
the packet, that is, the encapsulated transport layer PDU, remains unchanged
during the network layer processes.
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IPv4 Packet
•
IPv4 has been in use since 1983 when it was deployed on the Advanced Research Projects Agency Network (ARPANET), which was the precursor to the Internet. The Internet is largely based on IPv4, which is still the most widely-used network layer protocol.
•
An IPv4 packet has two parts:
•
IP Header - Identifies the packet characteristics.
•
Payload - Contains the Layer 4 segment information and the actual data.
•
an IPv4 packet header consists of fields containing important information about the
packet. These fields contain binary numbers which are examined by the Layer 3 process.
The binary values of each field identify various settings of the IP packet.
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IPv4 Packet
• Version - Contains a 4-bit binary value identifying the IP packet version. For IPv4 packets, this field is always set to 0100.
• Differentiated Services (DS) - Formerly called the Type of Service (ToS) field, the DS field is an 8-bit field used to determine the priority of each packet. The first 6 bits identify the Differentiated Services Code Point (DSCP) value that is used by a quality of service (QoS) mechanism. The last 2 bits identify the explicit congestion notification (ECN) value that can be used to prevent dropped packets during times of network congestion.
• Time-to-Live (TTL) - Contains an 8-bit binary value that is used to limit the lifetime of a packet. It is specified in seconds but is commonly referred to as hop count. The packet sender sets the initial time-to- live (TTL) value and is decreased by one each time the packet is processed by a router, or hop. If the TTL field decrements to zero, the router discards the packet and sends an Internet Control Message Protocol (ICMP) Time Exceeded message to the source IP address. The traceroute command uses this field to identify the routers used between the source and destination.
• Protocol - This 8-bit binary value indicates the data payload type that the packet is carrying, which enables the network layer to pass the data to the appropriate upper-layer protocol. Common values include ICMP (1), TCP (6), and UDP (17).
• Source IP Address - Contains a 32-bit binary value that represents the source IP address of the packet.
• Destination IP Address - Contains a 32-bit binary value that represents the destination IP address of the packet.
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IPv4 Header Fields
•
The remaining fields are used to identify and validate the packet, or to reorder a fragmented packet.
•
The fields used to identify and validate the packet include:
• Internet Header Length (IHL) - Contains a 4-bit binary value identifying the number of
32-bit words in the header. The IHL value varies due to the Options and Padding fields.
The minimum value for this field is 5 (i.e., 5×32 = 160 bits = 20 bytes) and the maximum value is 15 (i.e., 15×32 = 480 bits = 60 bytes).
• Total Length - Sometimes referred to as the Packet Length, this 16-bit field defines the
entire packet (fragment) size, including header and data, in bytes. The minimum length packet is 20 bytes (20-byte header + 0 bytes data) and the maximum is 65,535 bytes.
• Header Checksum - The 16-bit field is used for error checking of the IP header. The
checksum of the header is recalculated and compared to the value in the checksum field. If the values do not match, the packet is discarded.
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A router may have to fragment a packet when forwarding it from one medium to another medium that has a smaller MTU. When this happens, fragmentation occurs and the IPv4 packet uses the following fields to keep track of the fragments:
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Identification - This 16-bit field uniquely identifies the fragment of an original IP packet.
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Flags - This 3-bit field identifies how the packet is fragmented. It is used with the Fragment Offset and Identification fields to help reconstruct the fragment into the original packet.
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Fragment Offset - This 13-bit field identifies the order in which to place the packet fragment in the reconstruction of the original unfragmented packet.
•
Note: The Options and Padding fields are rarely used and beyond the scope of this
chapter.
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Sample IPv4 Headers
• Figure displays the contents of packet number 2 in this sample capture. Note that the Source is listed as 192.168.1.109 and the Destination is listed as 192.168.1.1. The middle window contains
information about the IPv4 header, such as the header length, total length, and any flags that are set.
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•
Figure displays the contents of packet number 8 in this sample capture. This is an HTTP packet. Also notice the presence of information beyond the TCP section.
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•
Finally, Figure displays the contents of packet number 16 in this sample capture. The sample
packet is a ping request from host 192.168.1.109 to host 192.168.1.1. Notice how there is no
TCP or UDP information because this is an Internet Control Message Protocol (ICMP) packet.
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IPv6 Packet
• Through the years, IPv4 has been updated to address new challenges. However, even with changes, IPv4 still has three major issues:
• IP address depletion
• Internet routing table expansion
• Lack of end-to-end connectivity
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Introducing IPv6
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Increased address space
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Improved packet handling
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Eliminates the need for NAT
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Integrated security
•
4 billion IPv4 addresses 4,000,000,000
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340 undecillion IPv6 addresses
340,000,000,000,000,000,000,000,000,000,000,000,000
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Encapsulating IPv6
•
One of the major design improvements of IPv6 over IPv4 is the simplified IPv6 header.
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The IPv4 header consists of 20 octets (up to 60 bytes if the Options field is used) and 12 basic header fields, not including the Options field and Padding field.
•
The IPv6 header consists of 40 octets (largely due to the length of the source and destination IPv6 addresses) and 8 header fields (3 IPv4 basic header fields and 5 additional header
fields).
•
Figure 1 shows the IPv4 header structure. As shown in the figure, for IPv6, some fields have remained the same, some fields from the IPv4 header are not used, and some fields have changed names and positions.
•
In addition, a new field has been added to IPv6 that is not used in IPv4. The IPv6 simplified header is shown in Figure
•
The IPv6 simplified header offers several advantages over IPv4:
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Better routing efficiency for performance and forwarding-rate scalability
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No requirement for processing checksums
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Simplified and more efficient extension header mechanisms (as opposed to the IPv4 Options field)
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A Flow Label field for per-flow processing with no need to open the transport inner
packet to identify the various traffic flows
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IPv6 Packet Header
• Version - This field contains a 4-bit binary value identifying the IP packet version. For IPv6 packets, this field is always set to 0110.
• Traffic Class - This 8-bit field is equivalent to the IPv4 Differentiated Services (DS) field. It also
contains a 6-bit Differentiated Services Code Point (DSCP) value used to classify packets and a 2-bit Explicit Congestion Notification (ECN) used for traffic congestion control.
• Flow Label - This 20-bit field provides a special service for real-time applications. It can be used to inform routers and switches to maintain the same path for the packet flow so that packets are not reordered.
• Payload Length - This 16-bit field is equivalent to the Total Length field in the IPv4 header. It defines the entire packet (fragment) size, including header and optional extensions.
• Next Header - This 8-bit field is equivalent to the IPv4 Protocol field. It indicates the data payload type that the packet is carrying, enabling the network layer to pass the data to the appropriate upper- layer protocol. This field is also used if there are optional extension headers added to the IPv6
packet.
• Hop Limit: - This 8-bit field replaces the IPv4 TTL field. This value is decremented by one by each router that forwards the packet. When the counter reaches 0 the packet is discarded and an ICMPv6 message is forwarded to the sending host, indicating that the packet did not reach its destination.
• Source Address - This 128-bit field identifies the IPv6 address of the sending host.
• Destination Address - This 128-bit field identifies the IPv6 address of the receiving host.
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IPv4 Addressing
IPv4 Network Addresses
•
IPv4 Address Structure
•
Binary notation refers to the fact that computers communicate in 1s and 0s
•
Converting binary to
decimal requires an
understanding of the
mathematical basis of a
numbering system –
positional notation
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Binary Number System
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Converting a Binary Address to Decimal
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Converting from Decimal to Binary
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Network Portion and Host Portion of an Address
•
To define the network and host portions of an address, a devices use a separate 32-bit pattern called a subnet mask
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The subnet mask does not actually contain the network or host portion of an IPv4 address, it just says where to look for these portions in a given IPv4 address
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Legacy Classful Addressing
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Classless Addressing
• Formal name is Classless Inter-Domain Routing (CIDR, pronounced “cider)
• Created a new set of standards that allowed service providers to allocate IPv4 addresses on any
address bit boundary (prefix length) instead of only by a class A, B, or C address
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Subnetting IP Address
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Reasons for Subnetting
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Large networks need to be segmented into smaller sub-networks, creating smaller groups of devices and services in order to:
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Control traffic by containing broadcast traffic within subnetwork
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Reduce overall network traffic and improve network performance
•
Subnetting - process of segmenting a network into multiple smaller network spaces called subnetworks or Subnets.
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Communication Between Subnets
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A router is necessary for devices on different networks and subnets to communicate.
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Each router interface must have an IPv4 host address that belongs to the network or subnet that the router interface is connected to.
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Devices on a network and subnet use the router interface attached to their LAN as their default gateway.
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IP Subnetting is FUNdamental
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Basic Subnetting
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▪ Borrowing Bits to Create Subnets
▪ Borrowing 1 bit 21 = 2 subnets
Subnet 1
Network 192.168.1.128-255/25 Mask: 255.255.255.128
Subnet 0
Network 192.168.1.0-127/25 Mask: 255.255.255.128
Borrowing 1 Bit from the host portion creates 2 subnets with the same subnet mask
Subnetting Formulas
▪Calculate Number of Subnets
▪ Calculate Number of Hosts
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There are two considerations when planning subnets:
• Number of Subnets required
• Number of Host addresses required
• Formula to determine number of useable hosts 2^n-2
• 2^n (where n is the number the number of host bits remaining) is used to calculate the number of hosts
• -2 Subnetwork ID and broadcast address cannot be
used on each subnet
How to Create Subnets
• Creating subnetworks is essentially the act of
taking bits from the host portion of the address and reserving them to define the subnet address
instead.
• Clearly this will result in fewer bits being available
for defining your hosts
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To create a subnet, we’ll start by fulfilling these three steps:
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Determine the number of required network IDs:
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One for each LAN subnet
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One for each wide area network connection
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Determine the number of required host IDs per subnet:
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One for each TCP/IP host
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One for each router interface
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Based on the above requirements, create the following:
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A unique subnet mask for your entire network
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A unique subnet ID for each physical segment
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A range of host IDs for each subnet
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•
When you’ve chosen a possible subnet mask for your network and need to determine the number of subnets, valid hosts, and the
broadcast addresses of a subnet that mask will provide, all you need to do is answer five simple questions:
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How many subnets does the chosen subnet mask produce?
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How many valid hosts per subnet are available?
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What are the valid subnets?
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What’s the broadcast address of each subnet?
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What are the valid hosts in each subnet?
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Example Creating 4 Subnets
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Borrowing 2 bits to create 4 subnets. 2
2= 4 subnets
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Example Creating 8 Subnets
•
Borrowing 3 bits to Create 8 Subnets. 2
3= 8 subnets
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LAN : VLSM & Non VLSM
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Traditional Subnetting Wastes Addresses
•
Traditional subnetting - same number of addresses is allocated for each subnet.
•
Subnets that require fewer addresses have unused (wasted) addresses. For example, WAN links only need 2 addresses.
•
Variable Length Subnet Mask (VLSM) or subnetting a subnet provides more efficient use of addresses.
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VLSM Subnetting Scheme
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CIDR Value
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Exercise (1)
R1
R2 R3
Network A
Network B Network C
500 host
64 host 16 host
?
?
?
?
?
?
Ne
t Network Range
Host Broadcast Subnet
Mask Number of host
IP : 100.10.0.0
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Exercise (2)
Given the address 192.168.20.19/28, which of the following are valid host addresses on this subnet? (choose two)
A. 192.168.20.29 B. 192,168.20.16 C. 192.168.20.17 D. 192.168.20.31 E. 192.168.20.0
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Exercise (3)
A national retail chain needs to design an IP addressing scheme to support a nationwide network. The company needs a minimum of 300 sub networks and a maximum of 50 host addresses per subnet. Working with only one class address, which of the following subnet mask will
support an appropriate addressing scheme? (choose two) A. 255.255.255.0
B. 255.255.255.128
C. 255.255.252.0
D. 255.255.255.224
E. 255.255.255.192
F. 255.255.248.0
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Exercise (4)
The network 172.25.0.0 has been divided into eight equal subnets. Which of the following IP addresses can be assigned to host in the third subnet if the IP subnet zero command is configure on the router? (choose three) A. 172.25.78.243
B. 172.25.98.16 C. 172.25.72.0 D. 172.25.94.255 E. 172.25.96.17 F. 172.25.100.16
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Routing
• How a Host Routes
• Host Forwarding Decision
• Default Gateway
• IPv4 Host Routing Table
• IPv4 Host Routing Entries
• Sample IPv4 Host Routing Table
• Sample IPv6 Host Routing Table
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Host Forwarding Decision
• Another role of the network layer is to direct packets between hosts. A host can send a packet to:
• Itself - A host can ping itself by sending a packet to a special IPv4 address of 127.0.0.1 which is referred to as the loopback interface. This loopback address is automatically assigned to a host when TCP/IP is running. The ability for a host to send a packet to itself using network
functionality is useful for testing purposes. Any IP within the network 127.0.0.0/8 refers to the local host.
• Local host - This is a host on the same network as the sending host. The hosts share the same network address.
• Remote host - This is a host on a remote network. The hosts do not share the same network address.
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• Whether a packet is destined for a local host or a remote host is determined by the IP address and subnet mask combination of the source (or sending) device compared to the IP address and subnet mask of the destination device.
Default Gateway
• The default gateway is the device that routes traffic from the local network to devices on remote
networks. In a home or small business environment, the default gateway is often used to connect the local network to the Internet.
• If the host is sending a packet to a device on a different IP network, then the host must forward the packet through the intermediate device to the default gateway. This is because a host device does not maintain routing information, beyond the local network, to reach remote destinations. The default gateway does. The default gateway, which is most often a router, maintains a routing table. A routing table is a data file in RAM that is used to store route information about directly connected network, as well as entries of remote networks the device has learned about. A router uses the information in the routing table to determine the best path to reach those destinations.
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• Hosts must maintain their own, local, routing table to ensure that network layer packets are directed to the correct destination network.
• The local table of the host typically contains:
• Direct connection
• Local network route
• Local default route
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IPv4 Host Routing Table
•
On a Windows host, the route print or netstat -r command can be used to display the host routing table. Both commands generate the same output.
The output may seem overwhelming at first, but is fairly simple to understand.
•
Entering the netstat -r command or the equivalent route print command, displays three sections related to the current TCP/IP network connections:
•
Interface List - Lists the Media Access Control (MAC) address and assigned interface number of every network-capable interface on the host including Ethernet, Wi-Fi, and Bluetooth adapters.
•
IPv4 Route Table - Lists all known IPv4 routes, including direct connections, local network, and local default routes.
•
IPv6 Route Table - Lists all known IPv6 routes, including direct connections, local network, and local default routes.
•
Note: Command output varies, depending on how the host is configured
and the interface types it has.
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• Notice the output is divided into five columns which identify:
• Network Destination - Lists the reachable networks.
• Netmask - Lists a subnet mask that informs the host how to determine the network and the host portions of the IP address.
• Gateway - Lists the address used by the local computer to get to a remote network destination. If a destination is directly
reachable, it will show as “on-link” in this column.
• Interface - Lists the address of the physical interface used to send the packet to the gateway that is used to reach the
network destination.
• Metric - Lists the cost of each route and is used to determine
the best route to a destination.
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IPv4 Host Routing Entries
• 0.0.0.0
The local default route; that is, all packets with destinations that do not match other specified addresses in the routing table are forwarded to the gateway. Therefore, all non-matching destination routes are sent to the gateway with IP address 192.168.10.1 (R1) exiting from the interface with IP address 192.168.10.10. Note that the final destination address specified in the packet does not change; rather, the host simply knows to forward the packet to the gateway for further processing.
• 127.0.0.0 – 127.255.255.255
These loopback addresses all relate to the direct connection and provide services to the local host.
• 192.168.10.0 - 192.168.10.255
These addresses all relate to the host and local network. All packets with destination addresses that fall into this category will exit out of the 192.168.10.10 interface.
• 192.168.10.0 - The local network route address; represents all computers on the 192.168.10.x network.
• 192.168.10.10 - The address of the local host.
• 192.168.10.255 - The network broadcast address; sends messages to all hosts on the local network route.
• 224.0.0.0
These are special multicast class D addresses reserved for use through either the loopback interface (127.0.0.1) or the host IP address (192.168.10.10).
• 255.255.255.255
The last two addresses represent the limited broadcast IP address values for use through either the loopback interface (127.0.0.1) or the host IP address (192.168.10.10). These addresses can be used to find a DHCP server before the local IP is determined.
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Sample IPv4 Host Routing Table
•
For example, if PC1 wanted to send a packet to
192.168.10.20, it would:
1. Consult the IPv4 Route Table.
2. Match the destination IP address with the 192.168.10.0
Network Destination entry to reveal that the host is on the same network (On- link).
3. PC1 would then send the packet toward the final destination using its local interface
(192.168.10.10).
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• If PC1 wanted to send a packet to a remote host located at 10.10.10.10, it would:
1. Consult the IPv4 Route Table.
2. Find that there is no exact match for the destination IP address.
3. Choose the local default route (0.0.0.0) to reveal that it should forward the packet to the 192.168.10.1 gateway address.
4. PC1 then forwards the packet to the gateway for using its local interface (192.168.10.10). The gateway device then
determines the next path for the packet to reach the final destination address of 10.10.10.10.
• Figure 2 highlights the matched route.
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Sample IPv6 Host Routing Table
• The output of the IPv6 Route Table differs in column headings and format due to the longer IPv6 addresses.
• The IPv6 Route Table section displays four columns which identify:
• If - Lists the interface numbers from the Interface List section of the netstat –r command. The interface numbers correspond to the network capable interface on the host, including Ethernet, Wi-Fi, and Bluetooth adapters.
• Metric - Lists the cost of each route to a destination. Lower numbers indicate preferred routes.
• Network Destination - Lists the reachable networks.
• Gateway - Lists the address used by the local host to forward packets to a remote network destination. On-link indicates that the host is currently connected to it.
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•
For example, the figure displays the IPv6 Route section generated by the netstat –r command to reveal the following network destinations:
•
::/0 - This is the IPv6 equivalent of the local default route.
•
::1/128 - This is equivalent to the IPv4 loopback address and provides services to the local host.
•
2001::/32 - This is the global unicast network prefix.
•
2001:0:9d38:953c:2c30:3071:e718:a926/128 - This is the global unicast IPv6 address of the local computer.
•
fe80::/64 - This is the local link network route address and represents all computers on the local link IPv6 network.
•
fe80::2c30:3071:e718:a926/128 - This is the link local IPv6 address of the local computer.
•
ff00::/8 - These are special reserved multicast class D addresses equivalent to the IPv4 224.x.x.x addresses.
•
Note: Interfaces in IPv6 commonly have two IPv6 addresses: a link local address and a
global unicast address. Also, notice that there are no broadcast addresses in IPv6. IPv6
addresses will be discussed further in the next chapter.
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Router Routing Tables
•
When a host sends a packet to another host, it will use its routing table to determine where to send the packet. If the destination host is on a remote network, the packet is forwarded to the address of a gateway device.
•
What happens when a packet arrives on a router interface? The router looks at its routing table to determine where to forward packets.
•
The routing table of a router stores information about:
•
Directly-connected routes - These routes come from the active router interfaces.
Routers add a directly connected route when an interface is configured with an IP address and is activated. Each of the router's interfaces is connected to a different network segment. Routers maintain information about the network segments that they are connected to within the routing table.
•
Remote routes - These routes come from remote networks connected to other routers. Routes to these networks can either be manually configured on the local router by the network administrator or dynamically configured by enabling the local router to exchange routing information with other routers using dynamic routing protocols.
•
The figure identifies the directly connected networks and remote networks of router R1.
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IPv4 Router Routing Table
•
A host routing table includes only information about directly-connected networks. A host requires a default gateway to send packets to a remote destination. The routing table of a router contains similar information but can also identify specific remote networks.
•
The routing table of a router is similar to the routing table of a host. They both identify the:
•
Destination network
•
Metric associated with the destination network
•
Gateway to get to the destination network
•
When a packet arrives at the router interface, the router examines the packet header to determine the destination network. If the destination network matches a route in the routing table, the router forwards the packet using the information specified in the routing table. If there are two or more possible routes to the same destination, the metric is used to decide which route appears on the routing table.
•
The figure shows the routing table of R1 in a simple network. Unlike the host routing
table, there are no column headings identifying the information contained in a routing
table entry. Therefore, it is important to learn the meaning of the different types of
information included in each entry.
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Directly Connected Routing Table Entries
• Two routing table entries are automatically created when an active router interface is configured with an IP address and subnet mask. The figure displays the routing table entries on R1 for the directly connected network 192.168.10.0. These entries were automatically added to the routing table when the GigabitEthernet 0/0 interface was configured and activated. The entries contain the following information:
• Route Source
The route source is labeled “A” in the figure. It identifies how the route was learned. Directly connected interfaces have two route source codes.
• C - Identifies a directly connected network. Directly connected networks are automatically created when an interface is configured with an IP address and activated.
• L - Identifies that this is a link local route. Link local routes are automatically created when an interface is configured with an IP address and activated.
• Destination network
The destination network is labeled “B” in the figure. It identifies the address of the remote network.
• Outgoing interface
The outgoing interface is labeled “C” in the figure. It identifies the exit interface to use when forwarding packets to the destination network.
• Note: Link local routing table entries did not appear in routing tables prior to IOS Release 15.
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• A router typically has multiple interfaces configured. The routing table stores information about both directly-connected and remote routes. As with directly connected networks, the route source identifies how the route was learned. For example, common codes for remote networks include:
• S - Identifies that the route was manually created by an administrator to reach a specific network. This is known as a static route.
• D - Identifies that the route was learned dynamically from another router using the Enhanced Interior Gateway Routing Protocol (EIGRP).
• O - Identifies that the route was learned dynamically from another router using the Open Shortest Path First (OSPF) routing protocol.
• Note: Other codes are beyond the scope of this chapter.
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Remote Network Routing Table Entries
• The figure displays a routing table entry on R1 for the route to remote network 10.1.1.0. The entry identifies the following information:
• Route source - Identifies how the route was learned.
• Destination network - Identifies the address of the remote
network.
• Administrative distance - Identifies the trustworthiness of the route source.
• Metric - Identifies the value assigned to reach the remote network. Lower values indicate preferred routes.
• Next-hop - Identifies the IP address of the next router to forward the packet.
• Route timestamp - Identifies when the route was last heard from.
• Outgoing interface - Identifies the exit interface to use to forward a packet toward the final
destination
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Next Hop Address
• A next hop is the address of the device that will process the packet next. For a host on a
network, the address of the default gateway (router interface) is the next hop for all packets that must be sent to another network. In the routing table of a router, each route to a remote network lists a next hop.
• When a packet destined for a remote network arrives at the router, the router matches the destination network to a route in the routing table. If a match is found, the router forwards the packet to the IP address of the next hop router using the interface identified by the route entry.
• A next hop router is the gateway to remote networks.
• For example, in the figure, a packet arriving at R1 destined for either the 10.1.1.0 or 10.1.2.0 network is forwarded to the next-hop address 209.165.200.226 using the Serial 0/0/0 interface.
• Networks directly connected to a router have no next-hop address, because a router can forward packets directly to hosts on these networks using the designated interface.
• Packets cannot be forwarded by the router without a route for the destination network in the routing table. If a route representing the destination network is not in the routing table, the packet is dropped (that is, not forwarded).
• However, just as a host can use a default gateway to forward a packet to an unknown
destination, a router can also be configured to use a default static route to create a Gateway of Last Resort. The Gateway of Last Resort will be covered in more detail in the CCNA Routing course.
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A Router is a Computer
•
There are many types of infrastructure routers available. In fact, Cisco routers are designed to address the needs of:
• Branch - Teleworkers, small business, and medium-size branch sites. Includes Cisco
800, 1900, 2900, and 3900 Integrated Series Routers (ISR) G2 (2nd generation).
• WAN - Large businesses, organizations, and enterprises. Includes the Cisco Catalyst
6500 Series Switches and the Cisco Aggregation Service Router (ASR) 1000.
• Service Provider - Large service providers. Includes Cisco ASR 1000, Cisco ASR 9000,
Cisco XR 12000, Cisco CRS-3 Carrier Routing System, and 7600 Series routers.
•
The focus of CCNA certification is on the branch family of routers. The figure displays the Cisco 1900, 2900, and 3900 ISR G2 family of routers.
•
Regardless of their function, size or complexity, all router models are essentially computers.
Just like computers, tablets, and smart devices, routers also require:
•
Operating systems (OS)
•
Central processing units (CPU)
•
Random-access memory (RAM)
•
Read-only memory (ROM)
•
A router also has special memory that includes Flash and nonvolatile random-access memory (NVRAM).
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Router CPU and OS
•
Like all computers, tablets, and smart devices, Cisco devices require a CPU to execute OS instructions, such as system initialization, routing functions, and switching functions.
•
The CPU requires an OS to provide routing and switching functions. The Cisco Internetwork Operating System (IOS) is the system software used for most Cisco devices regardless of the size and type of the device. It is used for routers, LAN switches, small wireless access points, large routers with dozens of interfaces, and many other devices.
•
The highlighted component in the figure is the CPU of a Cisco 1941 router with the heatsink
attached.
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Inside a Router
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Power Supply
Shield for Interface Card WIC
or High Speed WIC (HWIC) Shield for Interface Card WIC or High Speed WIC (HWIC)
Fan
Advanced Integration Module (AIM) option
that offloads processor- intensive
functions such as encryption from the
main CPU NVRAM CPU
SDRAM
Router Backplane
• A Cisco 1941 router includes the following connections:
• Console ports - Two console ports for the initial configuration and
command-line interface (CLI)
management access using a regular RJ-45 port and a new USB Type-B (mini-B USB) connector.
• AUX port - An RJ-45 port for remote management access; this is similar to the Console port.
• Two LAN interfaces - Two Gigabit Ethernet interfaces for LAN access.
• Enhanced high-speed WAN interface card (EHWIC) slots - Two slots that provide modularity and flexibility by enabling the router to support
different types of interface modules, including Serial, digital subscriber line (DSL), switch port, and wireless.
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Connecting to a Router
• Cisco devices, routers, and switches typically interconnect many devices. For this reason, these devices have several types of ports and interfaces. These ports and interfaces are used to connect cables to the device.
• The connections on a Cisco router can be grouped into two categories:
• Management ports - These are the console and auxiliary ports used to
configure, manage, and troubleshoot the router. Unlike LAN and WAN interfaces, management ports are not used for packet forwarding.
• Inband Router interfaces - These are the LAN and WAN interfaces configured with IP addressing to carry user traffic.
Ethernet interfaces are the most common LAN connections, while
common WAN connections include serial and DSL interfaces.
• The figure highlights the ports and interfaces of a Cisco 1941 ISR G2 router.
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LAN dan WAN Interfaces
• Router interfaces can be grouped into two categories:
• Ethernet LAN interfaces - Used for connecting cables that terminate with LAN devices, such as computers and switches. This interface can also be used to connect routers to each other.
Several conventions for naming Ethernet interfaces are popular: the older Ethernet,
FastEthernet, and Gigabit Ethernet. The name used depends on the device type and model.
• Serial WAN interfaces - Used for connecting routers to external networks, usually over a larger geographical distance. Similar to LAN interfaces, each serial WAN interface has its own IP address and subnet mask, which identifies it as a member of a specific network.
• The figure shows the LAN Interfaces and serial interfaces on the router.
